Special cooling systems in which metal in molten form is used as cooling medium are already known for a wide variety of applications such as the cooling of high-performance circuits, nuclear reactors or radiation sources for the X-ray range. Liquid metal has the advantage of very good heat conductivity and moreover has electromagnetic properties so that the pumps that are required for generating a flow of coolant can be small and can be implemented without moving parts and external to the coolant.
US 2012/0057680 A1 discloses a circulation of liquid metal for an x-ray radiation source in which the metal is held at high pressure and provided as an emitter material in droplet form to be impinged upon by an electron beam for plasma generation. However, the metal is not used as coolant here.
In short-wavelength emitting radiation sources, e.g., extreme ultraviolet (EUV) radiation sources, the components used to generate the plasma which are heated to a very high degree by the plasma generation are cooled with a liquid metal.
In the radiation source described in EP 2 198 674 B1, for instance, a rotating disk electrode is supplied with a liquid metal in a cooling circuit and the liquid metal is pumped into a cooling device after electrode contact. However, neither the enormous quantities of heat to be removed nor the temperature gradient to be maintained are sufficiently considered in the compact construction disclosed therein.
Another radiation source of this kind is disclosed in U.S. Pat. No. 7,427,766 B1. The radiation source works on the basis of a discharge plasma which is generated between two electrodes and which emits EUV radiation (in the range of 13.5 nm). In this case, the components which are highly heated during plasma generation are two rotating disk electrodes which are located opposite one another in such a way that a discharge region for the generation of a discharge plasma is formed between the electrodes at the location of the shortest distance between the circumferential faces of the electrodes. During rotation, the electrodes are partially dipped in each instance into a melt bath of liquid tin; the tin performs several functions, of which the most important are the forming of electrical contacts and the cooling of the electrodes. At the same time, however, the tin in U.S. Pat. No. 7,427,766 B1 is also emitter material for the generation of EUV radiation; the tin applied to the electrodes when rotating out of the melt bath is evaporated in the discharge region by means of a laser, and an electric discharge converts the evaporated tin into radiation-emitting plasma. The very high electrical outputs required for generating the discharge plasma are largely converted into waste heat which is absorbed by the electrodes and dissipated by immersion in the melt bath. However, U.S. Pat. No. 7,427,766 B1 does not state how the required cooling of the heated tin bath is to take place.
In this context, DE 10 2005 023 060 A1 discloses an extension of the tin bath by integrating the liquid metal, as a first coolant, into a cooling system. The cooling system has one heat exchanger through which the metal melt (as first coolant) flows outside an electrode housing directly encasing the electrodes in a cooling circulation via a connecting element of channels or pipes in another vessel part, containing a reservoir of the metal. Owing to the electrodes rotating at a high speed, the liquid metal is driven upwards from a lower feed channel of the connecting element at the slit-shaped immersion bath of the electrode housing to an upper return channel of the connecting element and transported through the adjacent reservoir in circulation. Excess metal quantities accumulate at a wiper and produce pressure in result to force the liquid metal through the return channel into the reservoir preferably via a filter or an oxide deposition chamber. Problems of the hot, aggressive metallic liquid after the heating by the electrode are neither mentioned nor considered.
For technical reasons relating to manufacture and for the sake of economy, stainless steel is usually used as material for the cooling vessel system; however, stainless steel is not sufficiently resistant to electrochemical corrosion of a metal melt (e.g., lithium, tin, etc.) and mechanical erosion of a heated metal.
General constructive solutions, how mechanical stress of vessel walls by dynamic metal melts can be reduced, can be found in the documents DE 1 800 959 C, DE 19 29 025 A or EP 1 854 571 B1, for example.
Steps are already known from US 2011/0101251 A1 for containing or preventing corrosion through liquid tin for an EUV radiation source in that structural component parts of an EUV radiation source whose surfaces come into direct contact with liquid tin are protected against corrosion by means of resistant coating. In addition to the extended life of the surfaces as a result of the corrosion-inhibiting coating of the structural component parts, the purity of the tin melt is also conserved for a longer time because it is not contaminated by the waste products of corrosion.
However, while corrosion of the inner vessel surfaces can indeed be suppressed by coating with TiN or other compounds known from US 2011/0101251 A1, this solution still has the disadvantage that, in cooling circuits with circulating metal melts, vessel walls which are inclined or curved in the direction of flow, e.g., pipe elbows, etc., and with which the highly heated metal melt collides mechanically are exposed to severe erosion in spite of the coating. The coating does not provide adequate protection at these locations, so that while the risks of ruptured pipes and contamination of the metal melt may be temporarily forestalled, they occur nonetheless.